1. Field of Invention
The invention relates to an optical mask and an illumination system. More particularly, the invention relates to an optical mask capable of adjusting the luminous flux and an illumination system.
2. Description of Related Art
Along with the advancement of technology, various types of display device distinct from the conventional cathode ray tubes are being heavily developed and promoted. These display devices include flat panel display devices such as, liquid crystal display (LCD), plasma display panel (PDP), organic light emitting diode (OLED), etc. Further, the projection types of display device, such as projectors, rear projection displays, etc. are also being heavily developed and promoted.
Currently the liquid crystal display and the plasma display panel have entered the mainstream and become the type of display device commonly used in daily life; however, a projection apparatus may provide a super-size screen (for example, greater than 52 inches) under a considerably lower cost. Hence, in the field of super-size display, the projection apparatuses remain un-replaceable. Further, as the fabrication technology continues to advance, the production of a projector gradually progresses in the direction of a lower cost and a lower price. Hence, the projection apparatus is applied in presentation in offices or in academy, the projection apparatus have also been extended to home theater system.
In the projection apparatus, an illumination system is adapted for providing illumination beam to illuminate a light valve. The light valve then converts the illumination beam into an image beam, and the image beam is projected by a projection lens on a screen to generate a screen image on screen. As the projection apparatus projects a completely black image, the light valve is unable to terminate all the lights to send to the lens and the screen. A basic brightness remains on the screen even with a completely black image. When the basic brightness is too high, the black image becomes not black enough. Hence, the contrast of the image picture projected by the projection apparatus decreases.